The invention relates to a bearing arrangement for machine-tool spindles.
Machine-tool spindles are mounted either in rolling bearings such as ball bearings or roller bearings; or in sliding bearings, in particular hydrostatic bearings.
Roller bearings have the advantage of low friction and hence low warming even at high speeds of rotation, but high local stresses between the rolling body and the rolling ring cause rough running, as a result of which vibrations are more easily transmitted or generated by the bearings themselves. Moreover, the circular running precision of roller bearings is, because of the roller bearing consisting of inner ring, rolling body and outer ring, extremely difficult to bring down to the low values required by modern machine tools. In order to avoid some of these disadvantages, it is known from DE 26 30740 A to dispose the outer rings of the roller bearings in hydrostatic bearings. This entails a substantial construction effort.
By contrast, very high circular running precisions can be achieved with a hydrostatic bearing, since only a few parts, specifically the shaft, the bearing bush and the housing bores, have to be produced. By lapping of the bores with a lapping mandrel, precise alignment, precise diameters and very high surface quality can be achieved. As a result of the lubricant film, vibration-damping running properties can be achieved, so that hydrostatic bearings can also be used for fine finishing work. Moreover, hydrostatic bearings are wear-free. A complete hydrostatic bearing system of this type is described in DE 43 24 140 A. As, however, a satisfactory hydrostatic mounting of a machine-tool spindle normally requires two axial and two radial hydrostatic bearings, a high pump rating is necessary in order to build up the necessary hydrostatic pressure in the bearing and achieve an adequate oil throughput without excessive warming. Moreover, cooling units are needed in order to keep the circulating lubricant oil and a uniform and acceptable temperature.
It is an object of the invention to provide a bearing arrangement that permits high circular running precision and good damping at a high cutting performance and high speeds of rotation without exhibiting the disadvantages of the known bearing arrangements.
On the basis of this object, a bearing arrangement for machine-tool spindles is proposed which, according to the invention, consists of a hydrostatic radial bearing in the region of the tool end of the spindle and at least one radial/axial roller bearing disposed parallel thereto. The hydrostatic radial bearing ensures high circular running precision, an extremely rigid bearing system, good damping and a high degree of freedom from damage caused by radial shocks.
The invention is based on the concept that the known properties of a hydrostatic bearing system are substantially desirable in the radial direction, whereas in the axial direction the guidance w provided by radial/axial bearings is sufficient. Moreover, the advantageous properties of a hydrostatic bearing system produce their effects substantially in the region of the tool end of the spindle, so that this is also the only region in which a hydrostatic radial bearing must be disposed.
If at least one radial/axial roller bearing is disposed parallel and adjacent to the hydrostatic radial bearing, while further radial roller bearings are disposed at the end of the spindle remote from the tool holder of the spindle, high speeds of rotation can be achieved without excessive increases in the pump and cooling ratings, since, by comparison with complete hydrostatic mounting with two radial and two axial bearings only a quarter of the pump and cooling rating is required. By contrast, the necessary oil supply to the roller bearings is negligible, since these are lubricated and cooled only by means of an oil mist that operates using small quantities.
Preferably, two parallel radial separable ball bearings for axial loads in both directions can be disposed adjacent to the hydrostatic radial bearing on the side of the hydrostatic radial bearing remote from the tool holder of the spindle, these roller bearings effecting, by comparison with the conventional roller bearings, a substantial improvement in the rigidity in the axial direction, as they can be produced with a pressure angle of 25xc2x0 instead of the customary 15xc2x0.
If the machine-tool spindle is drivable at different speeds of rotation, for example a low speed of rotation for heavy-duty cutting using large, heavy milling heads, for which good damping is important, a moderate speed of rotation for the finishing work, again using large, heavy milling heads, for which a high degree of circular running precision is necessary, and a high speed of rotation for the production of bores by means of drills which have a relatively small diameter or for simple milling work using small-diameter ball-headed milling cutters, it is advantageous if the bearing arrangement is so formed that the hydrostatic radial bearing is active only in the lower and, optionally, moderate rotational speed ranges, but not in the high rotational speed range. In this case, although the advantages of the hydrostatic mounting, such as good damping, high circular running precision and extremely rigid mounting, are lost in the high rotational speed range, those properties are not absolutely necessary when producing bores of up to 15 mm diameter and when milling using small-diameter ball-headed milling cutters.
The hydrostatic mounting can be made inoperative in a simple manner by interrupting the hydraulic oil supply. This can be achieved, for example, by an automatic switching apparatus which interrupts the hydraulic oil feed to the hydrostatic radial bearing above a pre-settable speed of rotation, the hydrostatic radial bearing preferably being simultaneously subjected to the action of compressed air in order to blow out the oil present in the hydrostatic radial bearing.